Particulate emissions from stationary sources are regulated by visible emission (opacity) and particulate mass concentration standards. Available white-light in-stack transmissometers are a reliable and relatively inexpensive means for monitoring opacity. The opacity-to-mass concentration relationship is strongly dependent on the size distribution of the scattering particles; therefore, opacity readings can be calibrated to provide reliable mass concentration readings only for sources where the particle size distribution is relatively constant. No similarly acceptable technique using transmissometry is currently available for monitoring mass concentration of particulate emissions.
Transmission data in the infrared energy range are shown in an article in APPLIED OPTICS, Vol. 16, No. 6 June 1977 entitled, "Infrared extinction spectra of some common liquid aerosols" by Hugh R. Carlon et al. In the article independency of the ratio of extinction coefficient to water content of fogs as a function of particle size for a particular wavelength used (12.5 .mu.m) is noted. The present invention, however, distinguishes over the disclosure of the Carlon et al article in the use of a transmissometer wavelength at which the extinction coefficient-to-mass concentration ratio for typical pollution aerosols is substantially independent of the particle size. Pollution particles differ from fog particles in that they are normally smaller, of higher refractive index (real part) and may be irregular in shape.